The invention relates to a sound body for a stringed instrument, having an upper backframe designed as a frame, to which a soundboard is adhesively bonded, on which there are provided at least one bridge extending approximately diagonally and a cast frame with a set of strings, and having a lower backframe which is arranged under the upper backframe and is connected thereto.
Sound bodies of the type described above are an essential element of upright pianos, grand pianos and other stringed instruments. The tuning of the instrument is predefined by the sound body. It is desirable for an instrument or sound body which has been tuned once to keep the tuning constant. The tuning of the sound body remains constant when the exact position of the elements of the sound body and, above all, the tension of the strings remain unchanged. The wooden components of the sound body, in particular of the soundboard, are, however, subject to dimensional fluctuations in the event of fluctuations in the climatic conditions in the room, through which the tuning of the sound body and of the instrument is then changed. Even small changes in the relative atmospheric humidity at the transition from the heating period to the summertime conditions in the room or from the transition between an air-conditioned practice room and a concert hall which is hardly heated at all have the effect of swelling or shrinkage of the wood and therefore a change to the set tuning.
The sound body described at the beginning can be taken from German Patent 375 023. Here, the upper backframe is formed by a frame made of grooved strips, which are led around the entire circumference of the soundboard, improve the effect of the sound and at the same time are intended to ensure protection against stressing. In this case, the narrow upper edge of the grooved strips forms the supporting surface for the soundboard to be glued on. The lower backframe consists of a hollow frame, which has a somewhat larger external contour than the upper backframe and is stiffened by two transverse spars spaced apart from each other.
In the event of fluctuations in the climatic conditions, it is primarily the dimensions of the soundboard which change, but also the dimensions of the upper backframe. Neither the soundboard nor the supporting element of the sound body, the upper backframe, therefore satisfy the preconditions of dimensional stability under climatic fluctuations. In attempts to improve the dimensional stability of sound bodies, it has been shown that an attempt to hamper or prevent the dimensional changes to the soundboard often leads to damage, especially to plastic deformations or cracks in the soundboard.